Evaporation Boat

There are various requirements that evaporation boats must meet. For the principle of resistance heating to work, they must be electrically conductive; at the same time, they need a sufficiently high degree of electric resistance to make heating them possible at all. Furthermore, they must be resistant to high temperatures and inert towards molten metal. To combine these properties, mixed ceramics are made from two or three components.

Two-component evaporation boats contain titanium diboride and BORON NITRIDE. Due to its bonding structure, titanium diboride offers excellent resistance to high temperatures and remains inert even to non-ferrous metal melts. It also improves the wettability of the evaporation boat through liquid aluminum and ensures a melting bath with an extensive surface. For example, when used for aluminum vapor deposition, it's completely wetted by liquid aluminum. In addition, titanium diboride is a very good electrical conductor. Boron nitride is added as a non-conductive material to set the electrical resistance of the evaporation boat.

Three-component mixed ceramics also include aluminum nitride. This material has very good thermal conductivity which makes it particularly suitable for large evaporation boats to ensure they heat up quickly and uniformly. Compared to two-component evaporation boats, the three-component versions have a shorter lifespan, with lower evaporation rates and less corrosion resistance. At the same time, the wettability of the evaporator is significantly improved and there's greater homogeneity of the vapor-deposited layers. Three-component evaporators are therefore primarily used in the manufacture of capacitors, since this requires a particularly high level of homogeneity in the coatings.

To perform thermal evaporation, you will need special equipment designed for this purpose. Here are the five key components

Evaporation source
This is where the material to be evaporated is placed. There are different types of evaporation sources, like evaporation boats and crucibles, depending on the material and how fast you want to evaporate it.

Vacuum chamber
The vacuum chamber creates a low-pressure environment, which is necessary for thermal evaporation. It should be able to reach high vacuum levels to keep the deposition process clean.

Heating system
The heating system heats up the material so that it evaporates. The method used depends on the material and its specific requirements.

Substrate holder
The substrate holder holds the surface on which the thin film will be deposited. It should be designed to fit different substrate sizes and allow for precise temperature control.

Power supply
The power supply provides electrical power to the heating system, controlling the heating rate and maintaining a stable temperature.

Choosing the right materials is important for successful thermal evaporation. Common materials used include metals, metal oxides, semiconductors, and organic compounds. Now that we have covered the equipment and materials, let's look at the step-by-step guide to the thermal evaporation process.

Thermal evaporation uses heat to turn a solid material into gas. This gas forms a thin film on a substrate when it cools down. The temperature needed for evaporation depends on the material's properties, like its melting point. Different materials need different heating methods to evaporate properly. Knowing your material's requirements is important for successful thermal evaporation.

Other factors, like the temperature of the substrate, the angle of deposition, and the chamber pressure, affect how fast the film forms and its quality. Adjusting these factors lets you control the thickness, adhesion, and structure of the film. Sometimes, you need to experiment to get the film you want.

Initial heating of the new boats also called “break in” must always be slow to get proper life from Boats. Reason? New boats will always have some absorbed moisture. If the boat is heated quickly in this condition, the moisture, converted into steam having much higher volume
Will have little time to escape and can cause cracks because of high pressure build up.

At the end of the cycle it must be ensured that all the Aluminum or any other metal being metalized has evaporated completely before heating is switched off. Otherwise residual metal will seep into the pores of boats; now differential cooling rates of boat and metal will crack the boat.

Care must be exercised that before the vacuum is broken, boats must have cooled down. This is because sudden contact with cold air can cause boat breakage. About 5 minutes is enough for boats to cool down.

Worn out copper clamps also are a big reason for boats not performing to their full capacity. Non uniform wear of clamp causes sparking between clamp and boat; this overheats and cracks the boat.

A good quality graphite foil is very important factor in enhancing the life of Boat. To save money, sometimes Metalizers use poor quality plumbing grade graphite foil. This foil contains many impurities and binders and offers resistance to the flow of current. This causes the ends of boat to overheat and break. Supervac supplies 100% pure graphite foil free of binders. This is made with the patented process of Exfoliation. This foil gives you the following outstanding benefits –
A) Offers no resistance to the flow of current.
B) Saves costly power.
C) Enhances the life of Boats.

There is one more major factor inter linking the life of evaporation boat with graphite foil. Good quality graphite foil has good compressibility. This allows a proper electrical contact in boats and clamp before heating begins and expansion of boats in heated condition. Poor quality graphite foil on the other hand will not allow expansion of boats in heated condition putting additional stress and subsequent breakage of boats.
 

Aluminum of purity 99% or more is required to avoid premature cracking. Impure aluminum will have impurities which will have different boiling temperatures; this will lead to an unstable pool of metal in boat cavity. This in turn results in non uniform heating of Evaporation Boat leading to thermal stress and breakage.

Age of aluminum wire also affects boat life. With age, aluminum oxidizes and Aluminum oxide thus formed causes spitting and excessive heating of Boats. Freshly made or “cleaned” wire works best.

Hardness of Aluminum should be maintained in proper range if required by tempering as a very hard wire is likely to puncture the boats.

Aluminum wire should be fed in such a way that it meets boat in the center widthwise and in between 1/3 to ½ lengthwise. This ensures proper pool formation and increases the life of boats. (I have written a blog post entirely on this topic in the past. You can read it here.)

Resistivity
A boat with low resistivity will easily let the current flow through it and therefore will not heat up. On the other hand, the highest resistivity is limited by the power supply of the device. Therefore, it is best if the boat has an optimal resistivity so that it can be heated slowly, allowing the aluminum to melt on it and form an even coating.

Wettability of the boat
This is a measure of how the molten metal (aluminum in most cases) behaves when it comes in contact with the ceramic boat. The better the wettability of the boat, the higher the quality of the coating. Impurities in the evaporation boat can lead to poor wetting. When the evaporation boat is not wetted properly, we face problems such as uneven spreading of the molten metal, puddles, splashing, etc. While high wettability is desirable, it comes at the expense of the life of the evaporation boat.

Corrosion resistance
When aluminum liquefies on the boat, it starts to react with the boron nitride in the boat to form aluminum nitride and boride compounds, which corrode the boat. Over time, cracks, grooves, and fissures form on the surface of the boat, leading to premature failure of the boat. But if the boat is made just for corrosion resistance, it will hamper wettability.

2-Component boats/3-component boats
Ceramic evaporation boats are of two types: 2-component boats and 3-component boats. 2-component boats are made of boron nitride and titanium diboride. 3-component boats are also available with boron nitride, titanium diboride and aluminum nitride for lower cost

Life on board
The importance of long life of the evaporation boat and uniformity of coating cannot be overemphasized. Good quality evaporation boat components not only resist corrosion but also allow for smooth and uniform spread of the molten metal

Price
It makes absolutely no sense to sacrifice the life of the evaporation boat and coating quality for a low-priced evaporation boat. While you may feel happy because you have saved some evaporation boat cost, you may actually have to pay a huge electricity bill. The machine downtime and the overall cost of the evaporation boat will also go up. On the contrary, it does not make economic sense to buy a very expensive evaporation boat.

Ease of use
For roll-to-roll coaters, the continuity and ease of use of the evaporation boat is a must. Otherwise, it will lead to a large inventory backlog and a capital freeze of lakhs of rupees. It is well known that a shortage of evaporation boats can bring the entire operation to a standstill.

Short Supplier Lead Times
Due to the potentially long lead times from suppliers, equipment operators are often forced to place large orders for evaporation boats months in advance. This can feel like a dagger hanging over the operator’s head, to say the least. It makes sense for a business if your evaporation boat supplier has a short lead time. This way, you won’t have to wait as long to receive your next batch of evaporation boats.